Batch processing of silicon wafers continues to be an important commercial process. Typically, a wafer tower, often called a boat, is placed within a vertical furnace and holds a large number of silicon wafers in a vertical stack with a horizontal orientation of the principal surfaces of the individual wafers for thermal processing of the wafer within the furnace. The thermal process may include flowing into the furnace a process gas, such as a precursor gas to deposit a layer on the wafers by chemical vapor deposition (CVD), for example, of silane to form a layer of polysilicon or additionally of nitrogen to form a layer of silicon nitride. Oxygen or nitrogen may be flowed in to thermally oxidize or nitride the wafers. Hydrogen may be used as a reducing agent for a high-temperature anneal. In other applications involving a non-reactive anneal of the wafers, the furnace may be filled with an inert gas. A high-temperature anneal in an inactive ambient may act as an implant anneal to activate implanted ions or to generally anneal the silicon wafer. On the other hand, CVD is typically performed at lower temperatures.
Quartz towers have long been used in such furnaces. However, as processing temperatures continue to rise, now often exceeding 1000° C. and even 1250° C., quartz has exhibited deleterious sagging at the higher temperatures and also is now considered a somewhat dirty material in view of the increasing purity levels required for advanced integrated circuits. Silicon carbide towers have been increasingly used for high-temperature processing. However, sintered silicon carbide is also a dirty material and CVD silicon carbide is expensive as a bulk material and is not completely effective as a surface coating over sintered silicon carbide.
Recently silicon ladder towers have been introduced for supporting silicon wafers, as disclosed by Boyle et al. in U.S. Pat. No. 6,450,346, incorporated herein by reference. By ladder tower is meant that each of the wafers is directly supported on respective teeth integrally formed on three or four legs of the tower held between two tower bases. At least the legs of these towers include structural members composed of elemental silicon, that is, substantially more than 50% or even more than 90% of all of the silicon atoms in the structural member is bonded to other silicon atoms and not to other elements. Elemental silicon is readily available in forms having purity levels above 99 atomic percent (at %). Silicon intended for the semiconductor industry is available in very pure forms having purity levels well above 99.99 at %. Thus, silicon used for structural members in support fixtures has the advantages over quartz and silicon carbide of very high purity and no differential thermal expansion relative to the supported silicon wafers.
A major problem facing high-temperature processing of silicon wafers for advanced integrated circuits is the creation of dislocations such as slip defects. Silicon towers have been observed to produce few or no such defects. However, an alternative approach applied to the more conventional quartz or silicon carbide towers uses wafer rings composed of quartz or silicon carbide supported by the legs of the tower and the rings in turn support the wafers along a substantial periphery of the wafer. There are a number of configurations, often referred to as ring boats, boat rings, or towers with wafer support rings. Often, the ring is welded or otherwise fixed to the tower. A closed-ring structure, however, has the problem of the difficulty of loading and unloading wafers to and from the rings. Furthermore, quartz and silicon carbide when used in rings magnify and continue to present their many problems related to differential thermal expansion and impurities. The coefficient of thermal expansion for silicon is 100 times greater than that for quartz and silicon exhibits substantially better thermal conductivity. Most prior art rings have a complex cross section which significantly increases the cost of fabricating them.